Chemistry of Life(Origin of the Elements)

ASTR 1420

Lecture #3

Section 3.2 & 5.3

There are 103 elements known to exist.Yet, all known life forms are mainly based on C, H, O, & N, and most advanced

organisms are using H2O! Why?Why shouldn’t there be life forms with iron skeleton and using methanol?

Origin of the ElementsAfter Big Bang, lightest elements (H, He, Li, Be) were created, but only for a limited time period (3 to 20 minutes).

92% H + 8% He and almost no other elements.

Creation of the Elements(Nuclear Synthesis)

• Hydrogen “burning” = the main energy source of stars over long, long time (e.g., for Sun, 10 billion years).

Hydrogen burning : 4 Hydrogens Helium + energy

US yearly energy consumption:1.107×1020 Joules= can light Sanford stadium for 100

billion years!= can be produced from a fusion of

mere 80lb of Hydrogen!

Cosmic abundance• Later stages of Nuclear fusions in Stars

Not much Li (H+He), Be, B (Be+H or He+Li)…

Cosmic Abundance• Big Bang + nuclearsynthesis by Stars created the current distribution of

chemical elements.o Hydrogen is the most abundant element in the Universeo Helium is the next abundant, but it is inerto Oxygen is the 3rd, Carbon is the 4th, Neon is the 5th, and Nitrogen is the 5th most

abundant elements. Neon is inert.

Therefore, if one wants to make something out of the current mix of elements in the Universe, the most efficient outcome will be a compound of H, O, C, N.

Chemical bonding• Elements want to have electrons in certain numbers! 2, 10, 18, etc. • Thus an element with 11 electrons wants to get rid of one electron while an

element with 7 electrons wants to accept one more electron from somewhere

Chemical reactions

Ionic bond : Example of a chemical bonding that makes up salt

covalent bond : sharing an electron (s)

Examples of Good pairs areH + F Hydrogen FlorideNa + Cl Sodium Chloride (salt)or 2H + O H2O

Cosmic AbundanceAfter many, many recycles

HydrogenHeliumCarbonNitrogenOxygenFluorineNeonSodiumMagnesiumAluminumSiliconPhosphorusSulfurArgon

Noble gases (he, Ne, Ar, etc.) do not participate in chemical reactions.

Earth Life is Carbon-Based• Life on Earth is made from more

than 20 chemical elements, but four elements make up about 96% of the mass.

(right figure) chemical composition of the human body by weight.

Oxygen mostly in H2O

Cell structures and their functions are due to Carbon Carbon-based life!

Any chemical compounds contain C “organic compound”

Why Carbon-based? Any other base element?

Requirements for the base element:o Abundant elemento Combine easily with itself and others

• Advantages of Carbon!Let’s consider all different ways of combining with

Hydrogen…1. Oxygen : H2O and H2O2

2. Nitrogen : NH3 (ammonia) and N2H2 (hydrazine)

3. Carbon : so many different ways… E.g., C90H84 or C167H336, …# of possible isomers for C167H336 = 9×1083

? Can this may be due to the difference in the maximum number of bondings for C (No=4), O (No=2), N (No=3)?

C60 (Bucky-balls)

Entirely made of Carbon. (superconductor material, etc.)

Why not Silicon?• Si has four “bonds” similar to Carbon. • Then why not Si-based life?

For example, CH4 (methane), SiH4 (silane).o Si-Si bond strength is ½ of the C-C bondo C-C, C-H, and C-O bond strengths are ~same,

but Si-H and S-O bonds are stronger than Si-Si hard to make a large complex structures (chain, rings, etc.)

o At low temperature, nearly all Si atoms will form SiO2 (quartz!) and this rock-forming material are very difficult to dissociate!

• Astronomical Evidence = Astronomers found no silicones or silanes in meteorites while many complex carbon-based molecules are seen in everywhere (meteorites, comets, interstellar clouds, etc.)

Although life based on silicon may be possible, it will be extremely uncommon at best!

Life based on carbon seems favored as the dominant kind of life in the Universe!

Is Water (H2O) the only choice of fluid for life in the Universe?

Solvents• Requirements for a good solvent:1. A solvent must be abundant!2. Remain liquid for a wide range of Temp!3. Dissolve a wide variety of chemical

compounds!

• Two other possibilities1. Ammonia (NH3)2. Methyl alcohol (CH3OH)

Is water (H2O) inevitable?

Solvent : dissolve other chemical compounds (to transport nutrients and wastes in the cells) other advanced alien life forms are very likely use solvent in their metebolism

Solvent T range for liquid Total range of T

water 0 to 100 C 100 C

ammonia -78 to -33 C 45 C

methyl alcohol -94 to +65 C 159 C

methane -182 to -164 C 18 C

ethane -183 to -89 C 94 C

At low temperature, chemical reactions are slower hence slower metabolism.

Unique characteristics of Water• Heat capacity and heat of vaporization help to regulate temperature of life.

• Surface tension : tendency of liquid to form a droplet! Water has the highest surface tension among all known liquids. Before cells evolved, surface tension would force some compounds together and would preserve the boundaries between inside/outside.

• Unique properties of Water: larger volume when frozen! Bursting pipes in winter by frozen waterFrozen ponds only on the surface!

• Non-water based life forms do not expand upon freezing space travel in hibernation!

Water Ammonia M. Alcohol

Heat Capacity 1 1.23 0.6

Heat of Vapor. 595 300 290

Requirements for Life

• Liquid Water as the best solvent for life1. A wider, higher range of temperature for being

liquid2. Solid water floats over liquid water3. Polar molecule dissolve certain types (polar)

of molecules only : cell membrane4. Water is the most common liquid (H is the

most abundant element and Oxygen is the 3rd most common element in the Universe)

5. Heat capacity regulate temperature

Environmental Requirements for Life

1. Must have a source of molecules2. Must have a source of energy3. Must have a liquid medium

Any worlds meet the #3 requirement, likely meet the first two requirements also. Why?

Conservatively…

A habitable world = only if it has a liquid water!

Source of Energy and Carbon

• Classification of metabolic sourceso Carbon

Heterotroph : by eating… Autotroph : self-producing…

o Energy Photo- : from sunlight Chemo- : either from food or inorganic

material

Classification Carbon source Energy source Examples

photoautotroph CO2 sunlight plants

chemoautotroph CO2 inorganic chemical in extreme environ.

photoheterotroph food sunlight bacteria/archaea

chemoheterotroph food food animals

In summary…

Important Concepts• Life on Earth relies on carbon

chemistry and water solvent not by accident but because of their inherent superiorities!

• Cosmic abundance• Superiority of Carbon as the base

element• Superiority of Water as solvent• Source of energy and carbon

Important Terms• Cosmic abundance (nuclear

synthesis)

Chapter/sections covered in this lecture : 5.3